In a power supply device disclosed in JP 2009-142146, the square of the detection value proportional to the value of the current flowing through the semiconductor switch connected to the wire is calculated and the value corresponding to the temperature rise determined from the squared detection value is compared with the abnormal determination value. Then, when the temperature rise value exceeds the abnormal determination value, the semiconductor switch is turned OFF.
In JP 2009-142146, the transient heat flow variation corresponding to the thermal variation in the semiconductor switch is represented as a logical expression, and the logical expression is calculated by digital computation.
However, the above technique disclosed in JP 2009-142146 requires digital circuits for achieving the logical expression. As a result, the protection circuit for the semiconductor switch is complicated in configuration.